“We have confirmed that we are not participating,” Bachmann spokeswoman Alice Stewart told ABC News late Thursday.

Her staff did not say specifically why she had decided to skip the Dec. 27 debate scheduled in Des Moines, Iowa. Bachmann has made numerous visits to Trump Tower in the past few months seeking the real estate mogul's endorsement. Earlier in the week she praised the debate and Trump for having a “voice” in the 2012 race.

Newt Gingrich and Rick Santorum both plan to attend the debate. Mitt Romney, Jon Huntsman, Rick Perry, and Ron Paul have declined to attend. Perry also made his decision on Thursday.

GOP front-runner Newt Gingrich and former U.S. Sen. Rick Santorum have agreed to participate in the debate.

Several major conservative groups and leaders have endorsed the debate vocally throughout the week, including:

The Tea Party Express became a sponsor Thursday, and urged candidates to attend.

“We are proud to join with Newsmax and other conservative organizations as a sponsor of this debate,” Tea Party Express Chairwoman Amy Kremer said. Kremer described Newsmax as “an indispensable part of the conservative media that keeps voters informed of the critical issues facing America.”

Note: Sign Up to RSVP to Join Donald Trump for the Newsmax ION Television Presidential Debate — Click Here Now!

Gingrich has said there is no downside for candidates to attend.

“We have to be open to new ways of doing things and new ways of approaching things,” Gingrich said Monday during a New York City visit to meet with Trump.

Gingrich then praised Trump and Newsmax, adding, “I thought it was great when [Donald Trump] agreed to do it. I think it’s great that Newsmax is helping launch it. It’s part of the process by which America governs itself."

Palin urged Republican presidential candidates Wednesday to take part in the debate, saying: “I think candidates should not be afraid in front of the nation no matter who the host of the debate is.

“What is a bit appealing about this idea of Trump hosting a debate is, consider the diverse audience that perhaps he can attract,” she said. “It’s important that independents and those who are not obsessed with inside-baseball partisan politics, which is most of us, we’d like to hear more of the message of each one of these candidates and perhaps Donald Trump will be able to attract a diverse demographic that maybe has not been as interested in this horse race thus far.”

And Reed’s pro-family Faith and Freedom Coalition, one of the nation’s most influential evangelical organizations, strongly endorsed the Trump-moderated debate. The group posted Reed's statement on its website:

“Newsmax is an outstanding news organization and I’m confident its GOP presidential candidate debate in Iowa broadcast on ION will be fair, balanced, professionally conducted and informative. I encourage members of Faith and Freedom Coalition in Iowa to attend. When it comes to opportunities for voters to hear directly from the candidates, I believe more is better. I’ll be watching on December 27.”

Norquist, president of Americans for Tax Reform, wrote a column questioning why mostly liberal-leaning news outlets have “framed” the conversation between Republican candidates and Republican voters in previous debates, and threw his support behind Trump and Newsmax’s efforts.

And Morris, a conservative author and Fox News analyst, went so far as to call Romney “foolish” for skipping the debate and urged all major candidates to join in.

“I think he is being foolish for saying no,” Morris told Fox News’ Bill O’Reilly Wednesday night. “I think that Romney should go. I think he will go.”

Morris said those who skip the debate are misjudging the broad reach such a debate would have.

“Originally I guess they were — they saw this as kind of limited TV exposure — but Newsmax, the group that's sponsoring it, has a wide reach and I suspect by the time the debate happens, it will be on a great many TV screens throughout the country,” Morris said.